[Mlir-commits] [mlir] Implement alternative decomposition for tanh (PR #85025)
llvmlistbot at llvm.org
llvmlistbot at llvm.org
Tue Mar 12 23:41:24 PDT 2024
https://github.com/srcarroll created https://github.com/llvm/llvm-project/pull/85025
None
>From c8eb4d84069d9929d414dddc321473915ee5e2da Mon Sep 17 00:00:00 2001
From: Sam <srcarroll314 at gmail.com>
Date: Wed, 13 Mar 2024 01:40:02 -0500
Subject: [PATCH] Implement alternative decomposition for tanh
---
.../Math/Transforms/ExpandPatterns.cpp | 40 +++++++++++--------
mlir/test/Dialect/Math/expand-math.mlir | 19 +++++----
2 files changed, 32 insertions(+), 27 deletions(-)
diff --git a/mlir/lib/Dialect/Math/Transforms/ExpandPatterns.cpp b/mlir/lib/Dialect/Math/Transforms/ExpandPatterns.cpp
index 989a3e5536ec66..1750171b81a10e 100644
--- a/mlir/lib/Dialect/Math/Transforms/ExpandPatterns.cpp
+++ b/mlir/lib/Dialect/Math/Transforms/ExpandPatterns.cpp
@@ -91,34 +91,40 @@ static LogicalResult convertCoshOp(math::CoshOp op, PatternRewriter &rewriter) {
}
/// Expands tanh op into
-/// 1) 1-exp^{-2x} / 1+exp^{-2x}, if x => 0
-/// 2) exp^{2x}-1 / exp^{2x}+1 , if x < 0
+/// 1-exp^{-2x} / 1+exp^{-2x}
+/// To avoid overflow we exploit the reflection symmetry `tanh(-x) = -tanh(x)`.
+/// We compute a "signs" value which is -1 if input is negative and +1 if input
+/// is positive. Then multiply the input by this value, guaranteeing that the
+/// result is positive, which also guarantees `exp^{-2x * sign(x)}` is in (0,
+/// 1]. Expand the computation on the input `x * sign(x)`, then multiply the
+/// result by `sign(x)` to retain sign of the real result.
static LogicalResult convertTanhOp(math::TanhOp op, PatternRewriter &rewriter) {
auto floatType = op.getOperand().getType();
Location loc = op.getLoc();
+ Value zero = createFloatConst(loc, floatType, 0.0, rewriter);
Value one = createFloatConst(loc, floatType, 1.0, rewriter);
- Value two = createFloatConst(loc, floatType, 2.0, rewriter);
- Value doubledX = rewriter.create<arith::MulFOp>(loc, op.getOperand(), two);
+ Value negTwo = createFloatConst(loc, floatType, -2.0, rewriter);
+
+ // Compute sign(x) = cast<float_type>(x < 0) * (-2) + 1
+ Value sign = rewriter.create<arith::CmpFOp>(loc, arith::CmpFPredicate::OLT,
+ op.getOperand(), zero);
+ sign = rewriter.create<arith::SIToFPOp>(loc, floatType, sign);
+ sign = rewriter.create<arith::MulFOp>(loc, sign, negTwo);
+ sign = rewriter.create<arith::AddFOp>(loc, sign, one);
- // Case 1: tanh(x) = 1-exp^{-2x} / 1+exp^{-2x}
- Value negDoubledX = rewriter.create<arith::NegFOp>(loc, doubledX);
+ // Normalize input to positive value: y = sign(x) * x
+ Value positiveX = rewriter.create<arith::MulFOp>(loc, sign, op.getOperand());
+
+ // Decompose on normalized input
+ Value negDoubledX = rewriter.create<arith::MulFOp>(loc, negTwo, positiveX);
Value exp2x = rewriter.create<math::ExpOp>(loc, negDoubledX);
Value dividend = rewriter.create<arith::SubFOp>(loc, one, exp2x);
Value divisor = rewriter.create<arith::AddFOp>(loc, one, exp2x);
Value positiveRes = rewriter.create<arith::DivFOp>(loc, dividend, divisor);
- // Case 2: tanh(x) = exp^{2x}-1 / exp^{2x}+1
- exp2x = rewriter.create<math::ExpOp>(loc, doubledX);
- dividend = rewriter.create<arith::SubFOp>(loc, exp2x, one);
- divisor = rewriter.create<arith::AddFOp>(loc, exp2x, one);
- Value negativeRes = rewriter.create<arith::DivFOp>(loc, dividend, divisor);
+ // Multiply result by sign(x) to retain signs from negative inputs
+ rewriter.replaceOpWithNewOp<arith::MulFOp>(op, sign, positiveRes);
- // tanh(x) = x >= 0 ? positiveRes : negativeRes
- Value zero = createFloatConst(loc, floatType, 0.0, rewriter);
- Value cmpRes = rewriter.create<arith::CmpFOp>(loc, arith::CmpFPredicate::OGE,
- op.getOperand(), zero);
- rewriter.replaceOpWithNewOp<arith::SelectOp>(op, cmpRes, positiveRes,
- negativeRes);
return success();
}
diff --git a/mlir/test/Dialect/Math/expand-math.mlir b/mlir/test/Dialect/Math/expand-math.mlir
index 6ee65b085dad1b..86ee5c8620472b 100644
--- a/mlir/test/Dialect/Math/expand-math.mlir
+++ b/mlir/test/Dialect/Math/expand-math.mlir
@@ -7,19 +7,18 @@ func.func @tanh(%arg: f32) -> f32 {
}
// CHECK-DAG: %[[ZERO:.+]] = arith.constant 0.000000e+00 : f32
// CHECK-DAG: %[[ONE:.+]] = arith.constant 1.000000e+00 : f32
-// CHECK-DAG: %[[TWO:.+]] = arith.constant 2.000000e+00 : f32
-// CHECK: %[[DOUBLEDX:.+]] = arith.mulf %arg0, %[[TWO]] : f32
-// CHECK: %[[NEGDOUBLEDX:.+]] = arith.negf %[[DOUBLEDX]] : f32
+// CHECK-DAG: %[[TWO:.+]] = arith.constant -2.000000e+00 : f32
+// CHECK: %[[VAL0:.+]] = arith.cmpf olt, %arg0, %[[ZERO]] : f32
+// CHECK: %[[VAL1:.+]] = arith.sitofp %[[VAL0]] : i1 to f32
+// CHECK: %[[VAL2:.+]] = arith.mulf %[[VAL1]], %[[TWO]] : f32
+// CHECK: %[[SIGN:.+]] = arith.addf %[[VAL2]], %[[ONE]] : f32
+// CHECK: %[[POSX:.+]] = arith.mulf %[[SIGN]], %arg0 : f32
+// CHECK: %[[NEGDOUBLEDX:.+]] = arith.mulf %[[POSX]], %[[TWO]] : f32
// CHECK: %[[EXP1:.+]] = math.exp %[[NEGDOUBLEDX]] : f32
// CHECK: %[[DIVIDEND1:.+]] = arith.subf %[[ONE]], %[[EXP1]] : f32
// CHECK: %[[DIVISOR1:.+]] = arith.addf %[[EXP1]], %[[ONE]] : f32
-// CHECK: %[[RES1:.+]] = arith.divf %[[DIVIDEND1]], %[[DIVISOR1]] : f32
-// CHECK: %[[EXP2:.+]] = math.exp %[[DOUBLEDX]] : f32
-// CHECK: %[[DIVIDEND2:.+]] = arith.subf %[[EXP2]], %[[ONE]] : f32
-// CHECK: %[[DIVISOR2:.+]] = arith.addf %[[EXP2]], %[[ONE]] : f32
-// CHECK: %[[RES2:.+]] = arith.divf %[[DIVIDEND2]], %[[DIVISOR2]] : f32
-// CHECK: %[[COND:.+]] = arith.cmpf oge, %arg0, %[[ZERO]] : f32
-// CHECK: %[[RESULT:.+]] = arith.select %[[COND]], %[[RES1]], %[[RES2]] : f32
+// CHECK: %[[POSRES:.+]] = arith.divf %[[DIVIDEND1]], %[[DIVISOR1]] : f32
+// CHECK: %[[RESULT:.+]] = arith.mulf %[[SIGN]], %[[POSRES]] : f32
// CHECK: return %[[RESULT]]
// -----
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